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There was/is a method of determining Latitude by two altitrudes, taken at
any time of the day, the interval or elapsed time between being measured
by watch or chronometer. This method is known as "Latitude by Double
Altitudes" and has little or nothing to do with Mr. Sumner.  It has
generally not been used for years, although in the past I have
experimented with it.

I am unable to take the time right now to explain this method, however,
its working may be found in most older navigation publications, i.e.,
Norie, etc.

On Mon, 28 Feb 2005 07:34:53 -0400 Jim Thompson 
writes:
> > -----Original Message-----
> > > on 2/11/05 7:22 AM, Jim Thompson at jim2@JIMTHOMPSON.NET wrote:
> > > Sumner opens his Introduction with this glorious single-sentence
> paragraph:
> > > "It is not so much the object of this work to present the
> navigator with
> a new method of 'Double Latitudes', as to afford him an accurate
> method of
> finding, by one Altitude of the Sun taken at any hour of the day,
> with the
> Chronometer time, the True Bearing of the Land, the Latitude, &c.,
> being,
> from any cause, uncertain; and to place him on his guard, when near
> a
> dangerous coast..."
>
> On re-reading this paragraph, I think that I need to understand the
> significance of his phrase, "Double Latitudes".  If calculating a
> second
> solution for a sight based on an assumed latitude was well known
> practice by
> 1837 for other purposes, then that would have been the building
> block that
> allowed Sumner to creatively apply the technique to his problem, and
> thus
> allow him to stumble on the celestial line of position concept when
> he found
> himself off Small's Light.  I do not understand the concept of
> "Double
> Latitudes", however, so I might have that all wrong.  I tried a
> google
> search, but turned up nothing on that technique.  I found lots on
> "double
> altitude", but not "double latitude".  Does anyone wish to explain
> the
> concept?
>
> Jim Thompson
>

   

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